The chart shows that it's basically the permanent No. 1 photo app downloaded in the U.S. by iPhone users. iPhone users are a good proxy for both iOS and Android device users. Among all apps of any kind, Snapchat always hovers around the top 10. That's a considerable achievement. We pulled this data from App Annie, the app analytics company:

As the App Annie ranking shows no sign of a decline in Snapchat's popularity, we can assume the app is still growing robustly. It's growing far faster than Twitter, for instance. It might eventually catch Instagram, too.